1. Field of the Invention
This invention relates to a method and system for automated computerized analysis of sizes of hearts and lungs in digital chest radiographs.
2. Discussion of Background
Cardiac size is an important diagnostic information included in chest radiographs. Abnormal enlargement of the heart is often detected initially in reviews of these images. The conventional method of assessing cardiac enlargement is measurement of the cardiothoracic ratio (CTR) (see Sutton, "A Textbook of Radiology and Imaging," 4th Edition, Vol. 1, pp. 554-556 (Churchill Livington, 1987); and Burgener et al., "Differential Diagnosis in Conventional Radiology," pp. 259-292 (Georg Thieme Verlag, Thieme-Stratton, 1985)) which is the ratio of the transverse diameter of the cardiac shadow to the transverse diameter of the thorax at the highest level of the diaphragm (see Danzer, "The Cardiothoracic Ratio An Index of Cardiac Enlargement," Am. J. Med. Sci. 157:513-524, 1919).
Fuster et al. (Am. J. Card. 47:525-531, 1981) investigated the relationship between mortality and prognostic factors such as the CTR by following up patients with idiopathic dilated cardiomyopathy for 6 to 20 years. They found that the larger the CTR, the greater the probability of death. They also found that the mortality was 86% in patients with a CTR of 55% or more, compared to 40% in patients with a CTR below 55%. Hutsebaut et al. (Respiration 41:25-32, 1981) studied the relationship between hemodynamic characteristics and cardiac size in patients suffering from chronic obstructive lung disease. They found that a small heart, which is typically associated with pulmonary hyperinflation and emphysema, tends to be related to a low cardiac output. Gomez et al. (Cancer Treat. Rep. 67:1099-1103, 1983) reported on the relationship between heart size and function after radiation therapy to the mediastinum in patients with Hodgkin's disease. Edwards et al. (AJR 136:907-913, 1981) and Kortman et al. (AJR 143:533-535, 1984) provided improved radiographic techniques for measuring heart size and CTR in infants, and they studied the relationship between heart size in newborn infants and birth asphyxia. Lauder et al. (Br. Heart J. 38:1286-1290, 1976) measured the transverse cardiac diameter and transverse thoracic diameter of older men and women over a period of five years and reported that the CTR tended to be increased after five years because of a significant decrease in the transverse thoracic diameter with age. In PA (posterior-anterior) chest radiographs, a CTR of 50% is generally accepted as an upper limit for normal cardiac size. However, Nickol et al. (Br. J. Radiol. 55:399-403, 1982) who investigated the relationship between heart size and CTR for a large population of different ages and races, concluded that a single upper limit for the CTR was unsatisfactory, and they provided an appropriate ratio for each group. Kabala et al. (Br. J. Radiol. 60:981-986, 1987) measured heart size in anterior-posterior (AP) chest radiographs and compared it with measurements made on PA chest radiographs. They concluded that an upper limit of 55% for the CTR and, for the heart diameter, of 165 mm in males and 150 mm in females for AP chest radiographs provided useful indices for distinguishing between normal and abnormal heart size.
The concept of automated computer analysis of radiographic images dates back to the 1960's. The first attempt at automated determination of the CTR was probably that of Meyers et al. (Radiology 83:1029-1033 1964). They used the spatial signature from digitized chest images and determined the edges of the heart and lung from the first derivative of the signature. (See also Becker et al., IEEE Trans. Biomed. Eng. BME-11:67-72, 1964.) Hall et al. (Radiology 101:497-509, 1971) and Kruger et al. (IEEE Trans. Biomed. Eng. BME-19:174-186, 1972) developed an algorithm for automated diagnosis of rheumatic heart disease, with which they computed the CTR and other cardiac parameters. Their approach was to determine a cardiac rectangle from analysis of the signatures and their derivatives, and then to estimate the cardiac shadow by thresholding the image on the basis of analysis of the histogram. Sezaki et al. (IEEE Trans. Biomed. Eng. BME-20:248-253, 1973) developed an algorithm with which they could compute the CTR for about 1 sec to provide radiologists with a practical instrument with which patients with abnormal hearts could be detected automatically by analysis of mass-screening chest radiographs. Paul et al. (IEEE Trans. Biomed. Eng. BME-21:441-451, 1974) computed the total lung volume by analyzing AP and lateral chest images, in which they determined the cardiac boundary by using the Gaussian-weighted derivative edge detection technique.
Since digital radiographic images were not readily available in the past, these automated methods were not implemented for practical clinical uses, and serious attention was not paid until recently. However, digital images can be obtained relatively easily at present with a number of digital radiographic systems such as those used for computed radiography (see Sonoda et al., Radiology 148:833-838, 1983). Therefore, the present invention is directed to a new automated method for computing the parameters related to cardiac size, including the CTR, to provide radiologists with new, useful tools.